More than eight-in-10 head teachers said the pupil premium – worth £1.25bn next year – had either equalled or failed to make up for financial losses elsewhere, it emerged.

Just a quarter of schools claimed the money would make any difference to children’s education.

The results will be seen as a blow to Nick Clegg, the Deputy Prime Minister, who has repeatedly championed the pupil premium as proof that the Liberal Democrats are having a positive effect on the Coalition’s reform programme.

Today, the Department for Education insisted that the money had to be specifically used to “help schools to improve the attainment of disadvantaged pupils.”

But Russell Hobby, NAHT general secretary, said: “In today's climate it is simply redistributing funds in the system, not adding more."

The pupil premium is a fixed sum handed to English state schools for each poor pupil – those from households earning less than £16,000-a-year – on their books.

This year, heads received £488 per child, rising to £600 in 2012/13, in addition to the underlying schools’ budget.

But the NAHT report – based on a survey of more than 2,000 heads – found that a 32 per cent believed the cash merely equalled losses elsewhere in their budget and 53 per cent said it had not made up for existing funding shortfalls.

Under rules, primaries and secondaries with large numbers of children eligible for free lunches will receive more money, while others get less.

But figures released ahead of the NAHT’s annual conference in Harrogate on Friday showed just over a quarter of heads believe it will make a difference to pupils’ achievement.

More than a third of heads said it would make no difference, while a similar proportion were unsure, it emerged.

Heads were also asked what they thought the premium could pay for in reality.

Of the more than 1,000 answers given, the most popular was extra teaching assistants, teachers, support staff and teaching time, followed closely by additional support such as one-to-one or small group tuition.

Schools were also using the money to pay for resources such as books and computers, while others were spending it on school trips, extra-curricular activities for their poorest pupils and uniforms or lunches.

A DfE spokeswoman said: "The pupil premium has been designed specifically to help schools to improve the attainment of disadvantaged pupils.

"We have protected the overall schools budget in cash terms so there should be no need for schools to use the premium for anything other than this purpose.

"From September, schools will have to publish information showing how they have spent the premium and what the impact of that spend was. Schools tell us they have used this extra money to pay for catch-up lessons, one-to-one tuition or after-school clubs."

But Stephen Twigg, the Shadow Education Secretary, “The Government promised that the pupil premium would help the poorest pupils do better. But the reality on the ground is that it is merely a fig leaf for the biggest cuts to education funding since the 1950s.”